Shaft component for a joint endoprosthesis

ABSTRACT

The shank unit of a joint endoprosthesis to be implanted in a tubular bone has a hollow, cylindrical, rotationally symmetrical anchoring sub-assembly (2) with a longitudinal axis (1) and an articulating sub-assembly (3). The articulating subassembly (3) is separate and detachably fixed to the anchoring sub-assembly (2). The articulating sub-assembly (3) comprises a rotationally symmetrical base element (31), an axis of rotation (32) and a neck extension (33) mounted eccentrically relative to the base element (31) and subtending an angle α of 70° to the axis of rotation (32) to receive a joint ball (36). The base element of the articulating sub-assembly (3) can be mounted on the anchoring sub-assembly (2) in such a way that its axis of rotation (32) coincides with the longitudinal axis (1) and be detachably fixed to sub-assembly (2) at any angular position relative to the longitudinal axis (1), as a result of which the radial direction of the neck extension (33) is selectively variable to adjust the anterior torsion relative to the anchoring sub-assembly (2).

FIELD OF THE INVENTION

The invention concerns a shank unit for an endo-prosthesis to beimplanted into a tubular bone and including a hollow-cylindricalanchoring sub-assembly with a longitudinal axis and an articulatingsub-assembly.

BACKGROUND OF THE INVENTION

A large number of shank units especially for hip-joint protheses arealready available, however, they all incur the drawback that theanchoring sub-assembly to be implanted into the bone marrow canal isrigidly joined to its neck part projecting from it and ultimatelyreceiving the joint-ball. Accordingly, once the shank unit has beenimplanted, it is impossible to adjust the position of the neck relativeto the now stationary anchoring sub-assembly. As a result the anteriortorsion or the neck-shank angle (centrum collum diaphysary angle CCD)cannot not be adjusted subsequently.

The state of the art suffers from another drawback in that a substantialnumber of shank units must be kept on hand to meet the main requirementsand anatomical particulars (shank diameter, shank length, neck-shankangle, neck length) of a particular patient.

SUMMARY OF THE INVENTION

The object of the invention is to create an easily implanted shank unitallowing changing the neck geometry (anterior torsion, neck-stub angle,neck length, trochanter replacement) during the operation and even afterit without the need to remove or fit the already implanted anchoringsub-assembly of the shank unit.

Essentially, the advantages of the invention are the optimal matching ofthe shank unit to the anatomic particulars and the modular bearing beingas compact as possible.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and further developments are elucidated below by means ofan illustrative embodiment of a shank unit for the hip joint. Howeverthe invention also applies to shank units of other endoprostheses,illustratively for the shoulder and the fingers.

FIG. 1 is an exploded perspective of the shank unit of the invention,and

FIG. 2 is a longitudinal section of a shank unit of FIG. 1 implanted inthe femur.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The hip joint prosthesis shank unit shown in FIGS. 1 and 2 substantiallycomprises a hollow cylindrical and rotationally symmetrical anchoringsub-assembly 2 with a longitudinal axis 1 and of separate articulatingsub-assembly 3 detachably affixed to anchoring sub-assembly 2.

At its upper end 25 facing the articulating sub-assembly 3, theanchoring sub-assembly 2 comprises a serrated disk surface 24 mountedperpendicularly to the longitudinal axis 1 and a central borehole 22with an inner thread 23 of constant diameter. At the other end 26 to beinserted into the bone marrow cavity 61 of the femur 6, the anchoringsub-assembly 2 constricts in the form of telescoping, coaxial, hollowcylinder segments, and as a result allows optimal matching to theanatomy of the bone marrow cavity 61. Furthermore, the anchoringsub-assembly 2 comprises a structured outer surface in the form of athread 21 and perforations 27 enhancing the growth penetration of bonematerial. Instead of the perforations and the thread, or in addition tothem, the surface also may be fitted with longitudinal channels parallelto the longitudinal axis 1, or with a coating or surface etching. Suchstructures serve to improve bone bonding.

Thread 21 is made self-tapping for the bone and thereby the use of aseparate tap, as required with conventional protheses, is eliminated.

Essentially, articulating sub-assembly 3 has a rotationally symmetrical,annular base element 31 with an axis of rotation 32 and with apreferably conical neck extension 33 mounted eccentrically on baseelement 31 and subtending an angle α of 70° to the axis of rotation 32to receive a joint ball 36. The joint ball also may be rigidly joined tothe neck extension 33.

At its lower end facing anchoring sub-assembly 2, articulatingsub-assembly 3 comprises a serrated disk surface 34, perpendicular toits longitudinal axis 32, of which the teeth have the same angularspacing apart as the teeth of the serrated disk surface 24 of anchoringsub-assembly 2. Base element 31 also has a central borehole 35 the upperend of which is enlarged to receive the head of a screw. As a result,articulating sub-assembly 3 can be mounted by its base element 31 on theanchoring sub-assembly 2 so that its axis of rotation 32 coincides withthat of anchoring sub-assembly 2 and so that it can be detachablyaffixed to anchoring sub-assembly 2 by screw 4 passing through centralborehole 35 of articulating sub-assembly 3 in any angular positionrelative to the longitudinal axis 1 of sub-assembly 2. As a consequence,the radial direction of neck extension 33 is arbitrarily adjustable toset the anterior torsion with respect to anchoring sub-assembly 2.

For that purpose, screw 4 must have an outer thread matching the innerthread 23 of the anchoring sub-assembly 2. However, instead of this typeof affixation using a screw 4 as shown in FIG. 1, other designs also maybe employed, for instance plug-in or snap-in or bayonet connections.

In a particular embodiment of the invention, such as shown by FIGS. 1and 2, an additional part 5 serving as a trochanter replacement may beprovided between articulating sub-assembly 3 and anchoring sub-assembly2. Part 5 essentially comprises a rotationally symmetrical second baseelement 51 with an axis of rotation 52 and having a cylindrical casesegment 53 mounted eccentrically on to base element 51. At both itsupper and lower ends, facing articulating sub-assembly 3 and anchoringsub-assembly 2 respectively base element 51 has serrated disk surfaces54 matching the serrated disk surfaces 24 and 34. In this manner thispart 5 also can be detachably fixed in place in any radial positionrelative both to anchoring sub-assembly 2 and to articulatingsub-assembly 3.

This additional part 5 may be eliminated where a trochanter replacementis not needed.

Again, part 5 may be designed in such manner that it can serve as anextension between sub-assemblies 2 and 3. Such a design is especiallyappropriate for extra-long femur shanks reaching as far as the kneejoint. In such cases it is possible to provide a borehole with an innerthread in the anchoring sub-assembly end 26 which is farthest away fromarticulating sub-assembly 3 to receive and mount an affixing part or apart of a knee-joint prosthesis.

I claim:
 1. A shank unit for an endo joint prosthesis to be implanted in a tubular bone and comprisinga hollow, cylindrical anchoring sub-assembly (2) having a longitudinal axis (1) and being substantially rotationally symmetrical,said anchoring sub-assembly comprising a plurality of coaxial, hollow cylindrical segments enlarging toward an upper end of said sub-assembly, said segments having perforated outer surfaces; an articulating sub-assembly (3) detachably connectable to said upper end of said anchoring sub-assembly (2), said articulating sub-assembly includinga substantially rotationally symmetrical base element (31), an axis of rotation (32), and a neck extension (33) mounted eccentrically on said base element and subtending an angle of less than 90° with said axis of rotation (32) for receiving a joint ball (36); said base element of said articulating sub-assembly being detachably mountable on said on said anchoring sub-assembly (2) with the axes of said anchoring and articulated sub-assemblies coincident and with said articulated subassembly in any selected rotational angular position relative to said anchoring subassembly such that a radial position of said neck extension can be selected to adjust anterior torsion to a desired level.
 2. A shank unit according to claim 1 and including a screw (4) for detachably mounting said articulating sub-assembly to said articulating sub-assembly.
 3. A shank unit according to claim 2 wherein said anchoring sub-assembly includes exterior threads whereby said sub-assembly is self-tapping into the bone.
 4. A shank unit according to claim 2 wherein said anchoring sub-assembly includes a central, axial borehole having an inner thread of substantially constant diameter for receiving said screw.
 5. A shank unit according to claim 2 wherein said base element (31) comprises a central borehole having an enlarged portion to receive a head of said screw.
 6. A shank unit according to claim 1 wherein said upper end of said anchoring sub-assembly comprises an upper serrated surface lying in a plane substantially perpendicular to said longitudinal axis, and wherein said base element comprises a lower serrated surface to engage said upper serrated surface.
 7. A shank unit according to claim 6 wherein serrations of said upper and lower surfaces have the same angular spacings as each other.
 8. A shank unit according to claim 1 and further comprising a second base element between said first-mentioned base element of said articulating sub-assembly and said upper end of said anchoring sub-assembly, said second base element being substantially rotationally symmetrical about an axis of rotation and having upper and lower structured surfaces, said upper end of said anchoring sub-assembly and a lower surface of said first base element comprising surfaces structured to mate with said second base element.
 9. A shank unit according to claim 8 and further comprising a cylindrical case segment (53) eccentrically mounted on said second base element, and wherein said mating structured surfaces are serrated.
 10. A shank unit according to claim 1 wherein said neck extension subtends an angle of between 60° and 80° with said axis of rotation.
 11. A shank unit according to claim 10 wherein said neck extension subtends an angle of between 65° and 75° with said axis of rotation.
 12. A shank unit according to claim 1 wherein a lower end of said anchoring sub-assembly comprises a borehole having an internal thread for receiving an additional prosthesis or fastener. 